For accurate radiometric readings, how many pixels must the measurement area subtend?

To get an accurate radiometric measurement, an object must subtend
or cover at least a diameter of 10 pixels. With a 10 pixel-wide object,
the pixel in the center of the subtended area should be absorbing the
maximum amount of photon flux. The focus of a thermal lens is not
perfect; pixels further from the center of the object will be affected
by radiation sources enclosing the area of interest. This will lead to
poor radiometry accuracy and is not recommended by FLIR.